Ripple reduction circuit for use with a power supply

ABSTRACT

A ripple reduction circuit for use with an AC/DC power supply providing an output voltage to a load is presented. The ripple reduction circuit includes an input terminal for receiving the output voltage and a low pass filter. The low pass filter is used to filter an AC component of the output voltage to obtain a filtered DC voltage. The ripple reduction circuit generates a reference current based on the filtered DC voltage and a control voltage having an AC component in phase with the AC component of the output voltage.

TECHNICAL FIELD

The present disclosure relates to a ripple reduction circuit and a powersupply with power factor correction provided with the ripple reductioncircuit.

BACKGROUND

AC/DC power supplies transform an AC input into a DC output voltage.This is usually achieved using a rectifier coupled to a transformer,hence forming an isolated power supply. Isolated AC/DC switching powerconverters can be used to provide regulated power to an electronicdevice while providing galvanic isolation between the electronic deviceand an AC power source. The transformer provides galvanic isolation, andcomponents which are coupled with the primary winding are collectivelyreferred to as the primary side of the power converter circuit, whilecomponents which are coupled with the secondary winding are collectivelyreferred to as the secondary side of the power converter circuit. Theoutput provides a regulated voltage for an output load.

An important figure of merit of such power supplies is the power factordefined as the ratio of the real power transmitted to the load over theapparent power received by the converter from the input source.International organizations like the EU have setup regulations whichdefine the minimum power factor or maximum level of harmonics a devicemust have in order to be sold in the European market. Reactive circuitelements such as inductors and capacitors can lower the power factor ofthe converter. To improve the power factor, AC/DC power supplies areoften provided with a power factor correction (PFC) and referred to asPFC converters.

Existing AC/DC power supplies are limited by significant currentoscillations and/or relatively complex designs, hence limiting theirapplications.

It is an object of the disclosure to address one or more of the abovementioned limitations.

SUMMARY

According to a first aspect of the disclosure, there is provided aripple reduction circuit for use with an AC/DC power supply providing anoutput voltage to a load, the ripple reduction circuit comprising aninput terminal for receiving the output voltage; a low pass filteradapted to filter an AC component of the output voltage to obtain afiltered DC voltage; wherein the ripple reduction circuit is adapted togenerate a reference current based on the filtered DC voltage and togenerate a control voltage having an AC component in phase with the ACcomponent of the output voltage.

Optionally, the load is provided between a first output terminal and asecond output terminal, the ripple reduction circuit comprising a linearvoltage regulator adapted to regulate a voltage at the second outputterminal based on the control voltage.

Optionally, the linear voltage regulator is a low dropout regulator.

Optionally, the voltage at the second output terminal is equal to thecontrol voltage.

Optionally, the low dropout regulator comprises an operational amplifiercoupled to a switch.

Optionally, the ripple reduction circuit comprises a current mirrorcircuit adapted to generate the reference current, wherein the referencecurrent has a first current component and a second current component.

Optionally, wherein the first current component is fixed, and the secondcurrent component is variable based on an output of the low pass filter.

Optionally, wherein the current mirror circuit comprises a first switchcoupled to the low pass filter via a first resistance and a secondswitch coupled to the input terminal via a second resistance or via theload.

Optionally, wherein the current mirror circuit comprises a thirdresistance coupled to a voltage source configured to provide a referencevoltage, wherein the third resistance is coupled to a control terminalof the first switch and a control terminal of the second switch.

Optionally, wherein the low pass filter has a time constant greater thana predetermined ripple period of the output voltage.

According to a second aspect of the disclosure, there is provided anAC/DC power supply comprising a power factor correction convertercoupled to a ripple reduction circuit according to the first aspect.

Optionally, the power factor correction converter comprises a rectifiercoupled to transformer having a primary winding and a secondary winding,a switch coupled to the primary winding and a power factor controllercoupled to the switch, wherein the power factor correction converter isoperable in at least one of a constant voltage mode and a constantcurrent mode.

Optionally, the power factor correction converter is adapted to providea variable output voltage.

The AC/DC power supply according to the second aspect of the disclosuremay comprise any of the features described above in relation to theripple reduction circuit according to the first aspect of thedisclosure.

According to a third aspect of the disclosure there is provided a methodfor reducing ripples of an AC/DC power supply providing an outputvoltage to a load, the method comprising

-   -   receiving the output voltage from the AC/DC power supply;    -   filtering with a low pass filter an AC component of the output        voltage to obtain a filtered DC voltage;    -   generating a reference current based on the filtered DC voltage;        and    -   generating a control voltage having an AC component in phase        with the AC component of the output voltage.

Optionally, the load is provided between a first output terminal and asecond output terminal, the method comprising regulating a voltage atthe second output terminal based on the control voltage.

Optionally, the output voltage varies over an output range, and whereinripples of a load output voltage or a load output current aresubstantially reduced or cancelled across the output range.

Optionally, wherein the reference current has a first current componentand a second current component, wherein the first current component isfixed, and the second current component is variable based on an outputof the low pass filter.

Optionally, the method comprises subtracting the DC component of theoutput voltage to obtain the control voltage.

Optionally, the method comprises generating the control voltage usingthe reference current and the output voltage.

The method of the third aspect may share features of the first andsecond aspects, as noted above and herein.

According to a fourth aspect of the disclosure there is provided an LEDdriver comprising an AC/DC power supply according to the second aspectof the disclosure.

DESCRIPTION OF THE DRAWINGS

The disclosure is described in further detail below by way of exampleand with reference to the accompanying drawings, in which:

FIG. 1A is a diagram of a conventional AC/DC switching converter withpower factor correction;

FIG. 1B is a diagram illustrating the operation of the PFC converter ofFIG. 1 ;

FIG. 2 is a simulation of the output current and the output voltage ofthe converter of FIG. 1A;

FIG. 3 is a diagram of an AC/DC converter provided with a DC-DCconverter output stage according to the prior art;

FIG. 4 is a diagram of an AC/DC converter provided with a ripplecancellation converter according to the prior art;

FIG. 5 is a diagram of an AC/DC converter provided with a constantcurrent regulator, according to the prior art;

FIG. 6 is a flow chart of a method for reducing output oscillations ofan AC/DC power supply, according to the disclosure;

FIG. 7A is a diagram of an AC/DC power supply provided with a ripplereduction circuit according to the disclosure;

FIG. 7B is a diagram of the ripple reduction circuit of FIG. 7A;

FIG. 7C is a diagram illustrating the operation of the ripple reductioncircuit of FIG. 7B;

FIG. 8 is a simulation showing the output voltage and output current ofthe PCF converter of FIG. 7A when the ripple reduction circuit is notprovided at the output;

FIG. 9 is a simulation showing the waveforms of the circuit of FIG. 7Aprovided with the ripple reduction circuit obtained for an outputvoltage of 25V;

FIG. 10 is a simulation showing the waveforms of the circuit of FIG. 7Aprovided with the ripple reduction circuit obtained for an outputvoltage of 35V;

FIG. 11 is a diagram of another ripple reduction circuit;

FIG. 12 is a diagram of an LED driver comprising an AC/DC power supplyaccording to the disclosure.

DESCRIPTION

FIG. 1A illustrates a conventional AC/DC isolated switching converter100 with power factor correction. The PFC converter 100 has afull-bridge rectifier 110 coupled at one end to an AC voltage source 120and at the other another end to a transformer T 130 having a primarywinding N_(P) and a secondary winding N_(S). The primary winding N_(P)is coupled at one end to the rectifier 110 for receiving a rectifiedinput voltage Vin, and at the other end to ground via a switch S₀ and aresistance R. A primary side controller 140 is coupled to the switch S₀.The primary side controller 140 is configured to perform power factorcorrection by implementing constant current CC and/or constant voltageCV control. The primary side controller 140 is also referred to as PFCcontroller. The secondary side N_(s) of the transformer T is coupled toan output capacitor C via a diode D. To meet power factor regulations(ex: required P.F.>0.9), the converter 100 is designed with almost zerobulk capacitor (input or filter capacitor) after the bridge rectifier.

FIG. 1B is a diagram illustrating the operation of the PFC converter ofFIG. 1 . FIG. 1B shows the output voltage Vout and the output currentIout in CV mode and CC mode, respectively. Both the output voltage andthe output current show ripples. The converter output voltage and outputcurrent are regulated by the PFC controller 140 at the primary side.

FIG. 2 is a simulation showing the output current and the output voltageof the converter of FIG. 1A. The output voltage Vout has significantripple oscillations at twice the AC line frequency, for instance 2×50 Hzor 2×60 Hz. Such ripples may affect the control and functionality ofsome devices including LEDs, for instance leading to flickering.

FIG. 3 is a diagram of an AC/DC converter provided with a DC-DCconverter output stage according to the prior art. The converter 300 issimilar to the PFC converter 100 of FIG. 1 , however in this case aDC-DC converter is provided as second stage to provide stable andripple-free DC voltage.

FIG. 4 is a diagram of an AC/DC converter provided with a ripplecancellation converter (RCC) as described in U.S. Ser. No. 10/015,849.In this example the transformer has a two secondary windings providingtwo outputs. An active RCC Buck converter is used on the second outputto generate a reverse-phase ripple voltage. This approach achievesoutput ripple compensation.

FIG. 5 is a diagram of an AC/DC converter provided with a constantcurrent regulator, according to the prior art. The converter 500 issimilar to the PFC converter 100 of FIG. 1 , however in this case aconstant current CC regulator 550 is provided at the output. The CCregulator 550 is formed of an operational amplifier having its outputconnected to a transistor M. The circuit 500 suppresses output ripplesbut can only be operated under specified CC condition. Since the loadvoltage under regulated current might be much lower than the converteroutput, the transistor M may dissipate significant power. For instance,if the load voltage is 30V and the converter output is 40V, then thetransistor M would have to lose 10V through power dissipation.

FIG. 6 is a flow chart of a method for reducing output oscillations(ripples) of an AC/DC power supply, according to the disclosure. Thepower supply is used to provide an output voltage to a load.

At step 610 the output voltage is received from the AC/DC power supply.The output voltage has a DC component and an AC component also referredto as residual AC component, or ripple component.

At step 620 the AC component of the output voltage is filtered with alow pass filter to obtain a filtered DC component.

At step 630 a reference current is generated based on the filtered DCvoltage. For instance, the reference current may have a first currentcomponent and a second current component, the first current componentbeing fixed, and the second current component being variable based on anoutput of the low pass filter.

At step 640 a control voltage having an AC component in phase with theAC component of the output voltage is generated. For instance thecontrol voltage may be generated using the reference current and theoutput voltage.

Using the proposed method, a load voltage across the load can beprovided with no or greatly reduced voltage ripples.

FIG. 7A illustrates an AC/DC power converter for implementing the methodof FIG. 6 .

The power converter 700 includes a PFC converter 710 coupled to a ripplereduction circuit 720.

FIG. 7B is diagram of the ripple reduction circuit of FIG. 7A. Theripple reduction circuit 720 comprises a low pass filter 722, a currentmirror circuit 724 and a linear voltage regulator 726.

The low pass filter 722 is formed of an operational amplifier OPA2, tworesistances R1 and R2 and a capacitor C1. The operational amplifier OPA2has an inverting input couple to its output at node C, and anon-inverting input couple to ground via the capacitor C1 at node B. Theresistance R1 is provided between node A and node B, and the secondresistance R2 between node B and ground.

The current mirror circuit 724 includes a pair of transistors S1 and S2having a common gate terminal at node D. The transistor S1 has a drainterminal coupled to a resistance Rb, and a source terminal coupled toground. The transistor S2 has a drain terminal coupled a resistance Rcfgat node E, and a source terminal coupled to ground. The resistance Rcfgis coupled to node A. The gate of S1 and S2 is coupled to a referencevoltage Vref via resistance Rb1. The drain of S1 is coupled to the gateof S1, S2 at node D.

The linear voltage regulator 726 is implemented as a low dropoutregulator made of operational amplifier OPA1 and transistor M1. Theoperational amplifier OPA1 has an inverting input couple to node E, anon-inverting input couple to the drain terminal of M1, and an outputcoupled to the gate terminal of M1. The load is provided between a firstoutput terminal (positive terminal) and a second output terminal(negative terminal). In the circuit of FIG. 7B the linear voltageregulator 726 is provided at the second output terminal. It will beappreciated that in an alternative implementation the linear voltageregulator may be provided at the first output terminal.

In operation the PFC converter 710 generates an output voltage Vout.Since the primary side of the AC/DC converter is being operated in apower factor correcting mode, Vout has a relatively large ripplecomponent. The voltage Vout is received at node A. The low pass filterLPF circuit 722 filters out or averages the AC frequency component ofVout (ripple frequency component), to produce the DC voltage Vopa2 atnode C. This permits the DC component of the output voltage Vout to bedetected and translated through the current mirror circuit 724 with Rcfgestablishing a gain factor. The current mirror circuit 724 receivesVopa2 and generates a control voltage Vc at node E. The control voltageVc is then used as a reference voltage for the linear voltage regulator726. The control voltage Vc is defined by equation (1) below. The linearvoltage regulator 726 regulates the voltage Vf at the negative side ofthe load so that Vc=Vf.

In more detail, the current I_(m) through R_(cfg) is mirrored from thereference current I_(ref) through S1; therefore the control voltage Vccan be expressed as:Vc=V _(out)−(I _(ref) *R _(cfg))  (1)

The control voltage Vc can be adjusted to be consistent with differentVout values.

The linear voltage regulator 726 operates to deliver a constant voltageV_(M1) at node F. The voltage V_(M1) is regulated to follow Vc.

The control voltage Vc should be selected above the saturation voltagelevel, V_(sat_min), so that M1 operates in the saturation region.

Since V_(M1) is regulated to follow the Vc, V_(M1)=Vc. Consequently, theload voltage V_(load) can be expressed as:V _(load) =I _(m) *R _(cfg) =I _(ref) *R _(cfg)  (2)

The control voltage Vc can also be expressed as the sum of V_(sat_min)with the ripple voltage of Vout, V_(ripple) as:Vc=V _(sat_min) +V _(ripple)  (3)V _(out) =V _(sat_min) +V _(ripple) +V _(load)  (4)

The reference current I_(ref)=I_(Rb1)+I_(Rb), in which I_(Rb1) is thecurrent through R_(b1), and I_(Rb) is the current through R_(b).

The current R_(b1) is a fixed bias current equal to V_(ref)/R_(b1), andthe current I_(Rb) is a variable current equal to V_(opa2)/R_(b). SinceV_(opa2) is proportional to Vout, then I_(ref) varies dynamically. IfVout increases, then V_(opa2) increases and I_(ref) increases.

However the reference current Iref does not vary linearity with Vout.This is due to the gate voltage Vgate of current mirror which has aconstant value. The current I_(Rb1) is added to compensate for thisnon-linearity. In addition, I_(Rb1) also dilutes the effect fromremaining ripple component of Iref due to components of the low passfilter. The current I_(Rb1) (hence the corresponding valued of V_(ref)and R_(b1)) can be selected to allow the control voltage Vc to beconsistent with the selected Vout output range.

The output V_(opa2) of the low pass filter 722 is proportional toV_(out) and scaled down by a factor N. The time constantTc=((R₁*R₂)/(R₁+R₂))*C₁ is designed to be greater than the max rippleperiod of the power factor correction PFC converter output ripple. Forinstance it may be chosen to be 5 times greater.

For example Tc>50 mS and the LPF 722 passes frequencies below 20 Hz.This provides a steady input to the OPA2 while providing sufficientresponse time.

For applications including LED lighting and charger, the power converterdoes not require a very fast response.

FIG. 7C is a diagram illustrating the load voltage for different valuesof the output voltage. The oscillations (ripples) of the output voltageVout at the positive terminal of the load are in phase with theoscillations of the voltage V_(M1) at the negative terminal of the load.Consequently the oscillations of the voltage Vload across the load arecancelled or greatly reduced.

In a numerical example, to cover a typical V_(out) range of 25V˜35V,R_(cfg) and I_(ref) have the following values:R _(cfg)=(25V−VC)/I _(ref@25V)=(35V−VC)/I _(ref@35V)I _(ref@25V)=(V _(ref) −V _(gs))/R _(b1)+(25V/N−V _(gs))/R _(b)I _(ref@35V)=(V _(ref) −V _(gs))/R _(b1)+(35V/N−V _(gs))/R _(b)

Assuming that VC is set to 2.5V for output range, 25V˜35V, V_(gs) ofcurrent mirror is 1V, V_(ref)=5V, R_(b)=2 kΩ, R_(b1)=10.7 kΩ and N=10(i.e. R₁=9*R₂), one would obtain:R _(cfg)=20kΩI _(ref@25V)=1.124 mAI _(ref@35V)=1.624 mA

N is Vout scale down factor equal to R1+R2. In the above exampleR₁=9*R₂, and Vopa2=(R₂/(R₁+R₂)) V_(out)=V_(out)/10.

FIG. 8 is a simulation showing the waveforms of the output voltage Voutof the PCF converter 710, and the load current Iout when the ripplereduction circuit 720 is not provided at the output. For an outputvoltage Vout=25V, the load current Iout has a peak to peak ripple of 140mA.

FIG. 9 is a simulation showing the waveforms of the circuit of FIG. 7provided with the ripple reduction circuit 720. The simulation shows theoutput voltage Vout, the load current Iout, the control voltage Vc andthe voltage across the transistor M1. For an output voltage Vout=25V,the load current Iout has a peak to peak ripple of 30 mA. The voltage Vcis identical to the voltage V_(M1). The oscillations/ripples of Vc arein phase with the oscillations/ripples of V_(M1).

FIG. 10 is a simulation showing the waveforms of the circuit of FIG. 7provided with the ripple reduction circuit 720. The simulation shows theoutput voltage Vout, the load current Iout, the control voltage Vc andthe voltage across the transistor M1. For an output voltage Vout=35V,the load current Iout has a peak to peak ripple of 30 mA. The voltage Vcis identical to the voltage V_(M1). Therefore the peak-to-peak ripplecurrent is reduced ˜80% from originally (30 mA vs 140 mA).

The power converter of the disclosure permits to suppress output ripplesfrom the PFC converter. In addition it can be used over a relativelywide range of output voltages, hence for different load conditions.

The method and corresponding power converter of the disclosure can beused in a variety of applications. For instance the power circuit of thedisclosure may be integrated as part of an LED driver.

FIG. 11 is a diagram of another ripple reduction circuit. The ripplereduction circuit 1100 is similar to the ripple reduction circuit ofFIG. 7B and the same reference numerals have been used to representcorresponding components. In this example the linear voltage regulatorhas been removed. The current mirror circuit 1124 is similar to thecurrent mirror 724 but does not include the resistance Rcfg. Instead,the load is provided directly at node E, between Vout and VC. The sizeratio of the transistors S1 and S2 can be selected such that the loadcurrent I_(load) through S2 is a constant factor N′ times the referencecurrent I_(ref) through S1.

FIG. 12 is a diagram of an LED driver comprising an AC/DC power supplyaccording to the disclosure. In this example the circuit of FIG. 7 isused to power a semiconductor light source such an LED, LED string or anarray of LEDs. In operation the ripple voltage across the LEDs isminimised, hence improving the light output. The output voltage may bevaried to provide a degree of dimming of the light source. The LEDcurrent (load current) may be varied by changing the DC voltage appliedacross the LEDs.

A skilled person will therefore appreciate that variations of thedisclosed arrangements are possible without departing from thedisclosure.

Accordingly, the above description of the specific embodiments is madeby way of example only and not for the purposes of limitation. It willbe clear to the skilled person that minor modifications may be madewithout significant changes to the operation described.

The invention claimed is:
 1. A ripple reduction circuit for use with anAC/DC power supply providing an output voltage to a load, the ripplereduction circuit comprising: an input terminal for receiving the outputvoltage; a low pass filter adapted to filter an AC component of theoutput voltage to obtain a filtered DC voltage; and a current mirrorcircuit adapted to generate a reference current, wherein the referencecurrent is a sum of a first current component and a second currentcomponent, wherein the ripple reduction circuit is adapted to generatethe reference current based on the filtered DC voltage and to generate acontrol voltage having an AC component in phase with the AC component ofthe output voltage.
 2. The ripple reduction circuit as claimed in claim1, wherein the load is provided between a first output terminal and asecond output terminal, the ripple reduction circuit comprising a linearvoltage regulator adapted to regulate a voltage at the second outputterminal based on the control voltage.
 3. The ripple reduction circuitas claimed in claim 2, wherein the linear voltage regulator is a lowdropout regulator.
 4. The ripple reduction circuit as claimed in claim2, wherein the voltage at the second output terminal is equal to thecontrol voltage.
 5. The ripple reduction circuit as claimed in claim 3,wherein the low dropout regulator comprises an operational amplifiercoupled to a switch.
 6. The ripple reduction circuit as claimed in claim1, wherein the first current component is fixed, and the second currentcomponent is variable based on an output of the low pass filter.
 7. Theripple reduction circuit as claimed in claim 1, wherein the currentmirror circuit comprises a first switch coupled to the low pass filtervia a first resistance and a second switch coupled to the input terminalvia a second resistance or via the load.
 8. The ripple reduction circuitas claimed in claim 7, wherein the current mirror circuit comprises athird resistance coupled to a voltage source configured to provide areference voltage, wherein the third resistance is coupled to a controlterminal of the first switch and a control terminal of the secondswitch.
 9. The ripple reduction circuit as claimed in claim 1, whereinthe low pass filter has a time constant greater than a predeterminedripple period of the output voltage.
 10. An AC/DC power supplycomprising a power factor correction converter coupled to a ripplereduction circuit as claimed in claim
 1. 11. The AC/DC power supply asclaimed in claim 10, wherein the power factor correction convertercomprises a rectifier coupled to transformer having a primary windingand a secondary winding, a switch coupled to the primary winding and apower factor controller coupled to the switch, wherein the power factorcorrection converter is operable in at least one of a constant voltagemode and a constant current mode.
 12. The AC/DC power supply as claimedin claim 11, wherein the power factor correction converter is adapted toprovide a variable output voltage.
 13. A method for reducing ripples ofan AC/DC power supply providing an output voltage to a load, the methodcomprising: receiving the output voltage from the AC/DC power supply;filtering with a low pass filter an AC component of the output voltageto obtain a filtered DC voltage; using a current mirror circuit,generating a reference current based on the filtered DC voltage whereinthe reference current is a sum of a first current component and a secondcurrent component; and generating a control voltage having an ACcomponent in phase with the AC component of the output voltage.
 14. Themethod as claimed in claim 13, wherein the load is provided between afirst output terminal and a second output terminal, the methodcomprising regulating a voltage at the second output terminal based onthe control voltage.
 15. The method as claimed in claim 13, wherein theoutput voltage varies over an output range, and wherein ripples of aload output voltage or a load output current are substantially reducedor cancelled across the output range.
 16. The method as claimed in claim13, wherein the reference current has a first current component and asecond current component, wherein the first current component is fixed,and the second current component is variable based on an output of thelow pass filter.
 17. The method as claimed in claim 13, comprisingsubtracting the a DC component of the output voltage to obtain thecontrol voltage.
 18. The method as claimed in claim 13, comprisinggenerating the control voltage using the reference current and theoutput voltage.
 19. An LED driver comprising an AC/DC power supplywherein the AC/DC power supply comprises a power factor correctionconverter coupled to a ripple reduction circuit, the ripple reductioncircuit comprising: an input terminal for receiving an output voltagefrom the AC/DC power supply; a low pass filter adapted to filter an ACcomponent of the output voltage to obtain a DC voltage; and a currentmirror circuit adapted to generate a reference current, wherein thereference current is a sum of a first current component and a secondcurrent component, wherein the ripple reduction circuit is adapted togenerate the reference current based on the DC voltage and to provide acontrol voltage having an AC component in phase with the AC component ofthe output voltage.